Charge measurement

Abstract

An apparatus comprises a comparator that includes a first input, a second input and an output. The comparator is configured for measuring a difference in voltage between a source coupled to the first input and another source coupled to the second input, and providing information associated with the measured difference at the output. The apparatus also comprises a controllable current source coupled to the first input of the comparator and configured for supplying or drawing current. The apparatus also comprises a digital logic circuit that is configured for controlling an amount of current supplied or drawn by the controllable current source. The apparatus is configured for measuring a charge associated with an external source that is coupled to the first input of the comparator.

What is claimed is: 
     
       1. An apparatus comprising:
 a comparator operable to receive a first voltage corresponding to a first amount of charge associated with a first source; 
 a controllable current source coupled to the comparator, the controllable current source operable to provide a current; and 
 a digital logic circuit coupled to the comparator, the digital logic circuit operable to:
 accumulate information corresponding to a second amount of charge, wherein the second amount of charge is proportional to an amount of the current provided by the controllable current source, 
 determine whether a threshold value for the amount of the current is reached; and 
 based on determining that the threshold value is reached, output the information corresponding to the second amount of charge as an indication of the first amount of charge. 
 
 
     
     
       2. The apparatus of  claim 1 , wherein the controllable current source is operable to supply the current to the first source and draw the current from the first source, and wherein the digital logic circuit is operable to cause an adjustment to the current supplied to the first source or drawn from the first source. 
     
     
       3. The apparatus of  claim 1 , wherein accumulation of the information corresponding to the second amount of charge is based on an amount of time during which the current is provided by the controllable current source. 
     
     
       4. The apparatus of  claim 1 , wherein the comparator is operable to measure a difference between the first voltage and a second voltage associated with a second source, and
 wherein the controllable current source is operable to provide the current to the first source based on the difference. 
 
     
     
       5. The apparatus of  claim 4 , wherein determining that the threshold value for the amount of the current is reached comprises determining that the first voltage is within a preselected range of the second voltage. 
     
     
       6. The apparatus of  claim 4 , wherein the digital logic circuit comprises:
 a logic gate that is operable to output a high value in response to the first voltage associated with the first source exceeding the second voltage associated with the second source based on the amount of the current provided to the first source by the controllable current source; and 
 a control circuit that is operable to control, in response to the high value being output by the logic gate, the controllable current source to thereby cause an adjustment to the amount of the current provided to the first source. 
 
     
     
       7. The apparatus of  claim 6 , wherein the information includes a value and the digital logic circuit further comprises a digital accumulator that is operable to:
 store the value corresponding to the second amount of charge; and 
 output the stored value in response to the threshold value being reached. 
 
     
     
       8. The apparatus of  claim 7 , wherein the digital logic circuit further comprises a compensation circuit, wherein the compensation circuit is operable to:
 receive a signal from the control circuit; 
 in response to a determination of the amount of the current provided to the first source by the controllable current source based on the received signal, compute the second amount of charge that is proportional to the amount of the current provided to the first source by the controllable current source; and 
 provide an indication of the second amount of charge to the digital accumulator. 
 
     
     
       9. The apparatus of  claim 8 , wherein the compensation circuit is further operable to:
 adjust the amount of the current provided to the first source by the controllable current source based on a digital lookup table. 
 
     
     
       10. The apparatus of  claim 6 , wherein the digital logic circuit further comprises:
 a digital filter that is operable to filter information corresponding to the difference between the first voltage and the second voltage that is output by the comparator; and 
 a delay circuit that is operable to receive the filtered information from the digital filter and provide a delayed version of the filtered information to the logic gate, 
 wherein the logic gate is operable to output the high value based on comparing the filtered information received from the digital filter and the delayed version of the filtered information received from the delay circuit. 
 
     
     
       11. The apparatus of  claim 1 , wherein the first source includes a capacitor. 
     
     
       12. A method comprising:
 receiving, at a comparator, a first voltage corresponding to a first amount of charge associated with a first source; 
 providing, by a controllable current source coupled to the comparator, a current to the first source 
 accumulating, by a digital logic circuit coupled to the comparator, information corresponding to a second amount of charge, wherein the second amount of charge is proportional to the amount of the current provided to the first source by the controllable current source; 
 determining, by the digital logic circuit, whether a threshold value for the amount of the current is reached; and 
 based on determining that the threshold value is reached, outputting, by the digital logic circuit, the information corresponding to the second amount of charge as an indication of the first amount of charge. 
 
     
     
       13. The method of  claim 12 , wherein accumulating the information corresponding to the second amount of charge comprises:
 accumulating the information corresponding to the second amount of charge based on an amount of time during which the current is provided by the controllable current source. 
 
     
     
       14. The method of  claim 12 , further comprising:
 measuring, by the comparator, a difference between the first voltage and a second voltage associated with a second source; and 
 based on measuring the difference, adjusting, by the controllable current source, an amount of the current provided to the first source by the controllable current source. 
 
     
     
       15. The method of  claim 14 , wherein determining that the threshold value for the amount of the current is reached comprises determining that the first voltage is within a preselected range of the second voltage. 
     
     
       16. The method of  claim 14 , further comprising:
 outputting, by a logic gate, a high value in response to the first voltage associated with the first source exceeding the second voltage associated with the second source based on the amount of the current provided to the first source by the controllable current source; and 
 in response to the high value being output by the logic gate, controlling, by a control circuit, the controllable current source to thereby cause an adjustment to adjust the amount of the current provided to the first source. 
 
     
     
       17. The method of  claim 16 , wherein the information includes a value, the method further comprising:
 storing, by a digital accumulator, the value corresponding to the second amount of charge; and 
 outputting the stored value in response to the threshold value being reached. 
 
     
     
       18. The method of  claim 17 , further comprising:
 receiving, at a compensation circuit, a signal from the control circuit; 
 in response to a determination of the amount of the current provided to the first source by the controllable current source based on the received signal, computing the second amount of charge that is proportional to the amount of the current provided to the first source by the controllable current source; and 
 providing an indication of the second amount of charge to the digital accumulator. 
 
     
     
       19. The method of  claim 18 , further comprising:
 adjusting, by the compensation circuit, the amount of the current provided to the first source by the controllable current source based on a digital lookup table. 
 
     
     
       20. The method of  claim 16 , further comprising:
 filtering, by a digital filter, information corresponding to the difference between the first voltage and the second voltage that is output by the comparator; 
 in response to receiving the filtered information from the digital filter, providing, by a delay circuit, a delayed version of the filtered information to the logic gate; and 
 outputting, by the logic gate, the high value based on comparing the filtered information received from the digital filter and the delayed version of the filtered information received from the delay circuit. 
 
     
     
       21. The method of  claim 12 , wherein the first source includes a capacitor. 
     
     
       22. An apparatus comprising:
 a comparator operable to receive a first voltage corresponding to a first amount of charge associated with a first source; 
 a controllable current source coupled to the comparator, the controllable current source operable to provide a current; and 
 a digital logic circuit coupled to the comparator, the digital logic circuit operable to output, based on a determination that a threshold value for the current is reached, an indication of the first amount of charge, 
 wherein the comparator is operable to measure a difference between the first voltage and a second voltage associated with a second source, 
 wherein the controllable current source is operable to provide the current to the first source based on the difference, and 
 wherein the digital logic circuit is further operable to:
 accumulate information corresponding to a second amount of charge, wherein the second amount of charge is proportional to an amount of the current provided by the controllable current source; 
 determine whether a threshold value for the amount of the current is reached; and 
 based on determining that the threshold value is reached, output the information corresponding to the second amount of charge as the indication of the first amount of charge.